If you get a fixed price, then the tradesman will have to presume the worst and quote accordingly (i.e. a high price will be quoted). If you get an estimate, then the tradesman might estimate low based on the ideal situation, knowing that (s)he can always recoup any losses by asking for extras which depend on what's found as the job goes on. An honest person will not fiddle the extras. If the job is simple, then a fixed price quote should be fairly easy to produce. If the job looks potentially tricky, then an estimate with the reservation that extras might crop up is reasonable. It can be a difficult choice to make. A recommended decorator would be your best choice.